1. Field of the Invention
The present invention relates to a charged particle beam lithography system, a pattern drawing method using a charged particle beam, and a method of manufacturing a semiconductor device.
2. Related Background Art
A charged particle beam lithography system can draw a pattern at a high resolution on the order of the wavelength of charged particles (such as electrons or ions) that is shorter than the visible wavelengths. Unlike a mask drawing method using light exposure, however, this method has a problem in that the complete pattern is divided into small sections and then is directly drawn by a beam of such a divided pattern, so that the drawing in this charged particle beam lithography system takes a long time. Since such a charged particle beam lithography system has the characteristic of enabling the formation fine line patterns to a high level of precision, however, it is being developed as the next-generation technique after lithography by optical exposure methods, or as a powerful tool for the manufacture of small lots of varied devices such as application specific integrated circuits (ASICs). Methods of forming a direct pattern by an electron beam include a method of scanning the wafer entire surface with a small round beam while turning the beam on and off, and a VSB drawing method of drawing a pattern with an electron beam that has passed through a stencil aperture. An electron beam drawing technique has been disclosed as a development from the VSB drawing method (such as in Japanese Patent Laid-Open No. 6-290727, Japanese Patent Laid-Open No. No. 2000-173529, Jpn. J. Appl. Phys. Vol. 34 (1995) Pt. 1, No. 12B, J. Vac. Sci. Technol. B 15 (6), November/December 1997, and J. Vac. Sci. Technol. B 19 (6), November/December 2001) to enable rapid drawing in batches. In this method, a stencil is prepared in which is formed a pattern consisting of a plurality of blocks of a repeating pattern, and this stencil is used for selective drawing.
In the VSB method that is disclosed in Jpn. J. Appl. Phys. Vol. 34 (1995) Pt. 1, No. 12B, an electron beam that has been accelerated to a high acceleration is driven into a resist layer on a wafer to ensure an improved beam resolution. With this high-acceleration voltage method, a phenomenon called the proximity effect occurs in which the irradiating electron beam reflects from the various layers of thin films formed on the lower surface of the resist on the upper surface of a wafer and is again directed through the resist, which leads to blurriness of the pattern to be drawn and a deterioration in the resolution. In this electron beam lithography system of this high-acceleration voltage method, it is therefore essential to exert control to correct this proximity effect, necessitating large-scale systems not only for the electronic optical system but also for the control itself. As a result, the system becomes even more complex, leading to a further problem in that the precision effectively deteriorates. In addition, the use of highly accelerated electrons leads to a fear that the surface of the wafer will become damaged.
To solve this problem with the VSB method when using a high-acceleration voltage charged particle beam, an electron beam drawing method of an aperture style that uses an electron beam with a low-acceleration voltage has been proposed (such as in Japanese Patent Laid-Open No. 2000-173529).
However, both the electron beam lithography system disclosed in Jpn. J. Appl. Phys. Vol. 34 (1995) Pt. 1, No. 12B and the electron beam drawing apparatus disclosed in Japanese Patent Laid-Open No. 2000-173529 have a problem in that a crossover is formed between a second shaping aperture of the optical system and the wafer, at which the diameter of the electron beam becomes small in order to reduce beam blurriness and warping due to optical aberrations. More specifically, the electron beam lithography system disclosed in Jpn. J. Appl. Phys. Vol. 34 (1995) Pt. 1, No. 12B has a high-demagnification optical system of 1/36 and the electron beam drawing apparatus disclosed in Japanese Patent Laid-Open No. 2000-173529 has a demagnification optical system on the order of 1/10. This means that with respect to each apparatus the aperture angle of the beam starting at the second shaping aperture is extremely small, so that the crossover has dimensions of a few micrometers and this is the smallest beam diameter with each demagnification projection optical system.
If an optical system that forms a crossover of such a small beam diameter is employed in an electron beam drawing apparatus using a low-acceleration voltage, a problem arises in that the blurriness of the beam is increased by the space charge effect and thus the resolution deteriorates.
A common problem in the above-described apparatus is that secondary electrons generated from the surface of the wafer cause contamination and charging of the objective lens and the deflector in the vicinity thereof, which results in a further worsening and drifting of the beam blurriness. Note that the usage of the term “secondary electrons” in this document has a broader meaning that includes reflected electrons.